The present invention relates to a rotary screw compressor for oil-free air provided with inlet channel means and outlet channel means and having at least one compression stage, each stage including at least one rotor with at least one shaft journal mounted in oil-lubricated bearing means, which shaft journal is provided with seal means between the rotor and the bearing means, which seal means include at least three frictionless seal means surrounding the shaft journal and a plurality of annular chambers surrounding the shaft journal, which frictionless seal means and annular chambers are arranged in an alternating sequence along the shaft journal.
In such compressors handling oil-free air an effective seal between the working space and the bearings is necessary in order to avoid that oil-contaminated air from the bearing housing mixes with the compressed air which can be dry or contain water. The use of direct contact mechanical seals for that purpose results in high friction losses. This is avoided with the type of seal arrangement mentioned above where frictionless seals are used together with a blocking gas, e.g. air.
Seals of this type are disclosed in U.S. Pat. No. 3,975,123 and FR-A-2 569 780.
In U.S. 3,975,123 the seal arrangement has four frictionless seals with three annular chambers between them. Blocking air is supplied to the intermediate annular chamber of the seal and flows outward towards the outermost annular chamber, from which it is withdrawn together with oil leaking from oil-lubricated bearing means. The innermost annular chamber is inactive at full load and connected to inlet pressure at part load for supply of air.
In FR-A-2 569 780 a similar seal device is applied to a two stage air compressor, but in this case only two annular chambers are present. Also in this disclosure the supply and withdrawal of blocking air is such that the blocking air flows in the outward direction.
The relative localisation of the supply and withdrawal of the blocking air in the known devices has the consequence that oil-contaminated air from the roller bearings leaks to the withdrawal channel where it is mixed with the blocking air and leaking air from the compression stage. This causes a problem in that the withdrawn air has to be purified if it is led to the compressor inlet or to the ambient atmosphere.
The object of the present invention is to attain a seal arrangement of the kind in question in which the above described problem is overcome.